A．Because the United Nations wanted to reduce road accidents and economic costs.

　　B．Because two million people were killed in the accidents.

　　C．Because engineers wanted to design ten new concrete mixtures.

　　D．Because the United Nations made bridges of the new mixtures which could last for about 20 to 35 years.

　　42．What does the underlined word “This” in the sixth paragraph refer to?

　　A．Fly ash.

　　B．Portland cement.

　　C．Sand.

　　D．Chemical.

　　43．Which of the following statements is TRUE?

　　A．Ten new concrete bridges have already been built in the United States.

　　B．A new concrete bridge could last 50 more years than an ordinary concrete one.

　　C．People didn’t know how to build with cement until the late 1800s.

　　D．Water and salt won’t do any damage to bridges over time.

　　44．What can we conclude from the passage?

　　A．Engineers have to pay a lot of money to use the new bridge technology.

　　B．Pennsylvania State University is paying all the money for the research.

　　C．Bridges built with fly ash are cheaper than common bridges.

　　D．Fly ash is much more expensive than Portland cement.

　　45．What does the passage mainly tell us?

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